Linear data storage tape comprises a medium for storing large amounts of data, and typically comprises a plurality of data tracks that extend longitudinally along the tape. A common example comprises magnetic tape media, and a less common example comprises optical tape media. A tape head is employed for reading and/or writing data on the data tracks, and is typically shared between various data tracks or groups of data tracks, and is moved between tracks or groups of tracks in the lateral direction of the tape. In magnetic tape media, the tape head typically comprises a number of separate elements which read and/or write data with respect to a number of parallel tracks, and is provided with one or more separate servo read transducers, which are laterally offset from the read and/or write elements, so as to track follow a servo band and cause the read and/or write elements of the tape head to be guided along the data track or tracks. In optical tape media, the optical servo may be associated with an individual data track or a with a separate servo track.
One type of servo system for linear data storage tape is one in which separate servo bands are laterally positioned on the linear data storage tape. Each of the servo bands provides the servo guidance for a group of data tracks, and the servo transducer of the tape head is repositioned laterally within a servo band so the read and/or write elements access different data tracks, and is repositioned laterally to another servo band to access still further data tracks. In one example, the servo bands are spaced apart and the data tracks are located between the servo bands. To insure that the servoing is precise, two servo heads may be provided at either end of the tape head, straddling the data read and/or write elements. The lateral positioning may be obtained from either or both servo bands. The servo bands are encoded with essentially identical patterns for determining lateral position, such that the bands are substantially indistinguishable.
The lateral positioning of the tape head is typically accomplished by actuators, which may have mechanical or electromechanical components. Once the proper lateral positioning of the tape head has been accomplished, as the servo information being sensed by the servo transducer indicates, minor adjustments of the head to follow lateral movement of the tape or of the tracks on the tape may be made. During track following, sticking or other failure of the mechanical or electromechanical components can be ascertained from failure of the sensed servo information to show any correction. Similarly, lateral repositioning of the tape head to different tracks within the same servo band is accomplished by a continuous adjustment of position within the servo band. Hence, any sticking or other failure of the mechanical or electromechanical components can be ascertained by failure of the sensed servo information to show the desired movement.
However, the lateral repositioning of the head between the servo bands is typically conducted by a coarse actuator, which may have mechanical or electromechanical components, such as a stepper motor, and which typically operates in open loop without feedback. Thus, as the tape head is repositioned between the servo bands, there is no feedback from the servo information to indicate that the switch from one servo band to any other servo band was successful, and, if the servo bands are substantially indistinguishable, at the supposed completion of the lateral movement, the tape head may be positioned at the wrong servo band, and the servo information will not indicate an error.
One way of determining whether the lateral movement has caused the tape head to be positioned at the correct servo band, is to provide a separate “independent” sensor, for example, that determines the approximate lateral position of the head with respect to the tape. Such an independent sensor may comprise a coarse optical sensor that measures the physical position of the head. Such a coarse sensor cannot be used for track following, but provides a backup to the actual servo system. Such extra sensors add cost to a tape drive, which is always undesirable, if the extra cost can be avoided. Another example is shown by U.S. Pat. No. 6,169,640, in which timing based servo bands are longitudinally displaced or offset from each other, such that by simultaneously sensing two adjacent servo bands allows the servo system to determine longitudinal offset between servo bands, from which the data band location of the tape head can be determined. The system, however, requires that both servo bands be sensed simultaneously in order to establish a differentiation and make a determination.